The summary element is not a structural tag, it has its own display properties. It is meant to be a toggled visibility box for the details tag.

According to both the MDN and CanIUse, Chrome has fully implemented the summary tag, while Firefox has not. For an un-implemented tag type, the default behavior of most major browsers is to draw the element as a generic block-level element. In Firefox, then, using the summary tag is essentially the same as using the div tag. In Chrome; however, it may very well be rendered as a replaced element, which would mean (among other things) that you cannot override its display type.

EDIT: The summary tag is now implemented in Firefox as well, as of version 49.

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